This invention relates to a conveying unit for feeding portions of sheet material to a user machine.
More particularly, the present invention relates to a continuous belt conveying unit for feeding preshaped or punched pieces of cardboard or the like to an intermittent machine for packaging cigarettes into hinged lid packets.
As described in Italian patent No. 992.092 of the same applicant, a packaging machine of the aforesaid type may be associated with a feed and accumulation apparatus in which preshaped pieces are continuously fed into a column vessel, the lower end of which is disposed above an inlet station of a first conveyor, an outlet station of which is connected to an inlet station of a second conveyor constituting the inlet element for said packaging machine and forming an angle of 90.degree. to said first conveyor.
In a conveying unit for feeding an intermittent packaging machine and comprising the two said conveyors forming a contained angle of 90.degree., transfer of the preshaped pieces from the first to the second conveyor present numerous technical problems. In this respect, the transfer of the preshaped pieces from one conveyor to the other must take place at an extremely precise rate and with extremely narrow position tolerances. To satisfy these conditions, the engagement between the preshaped pieces and transfer members arranged to transfer them from the first to the second conveyor must be always perfect, and the position assumed by the preshaped pieces on entering the second conveyor must also be perfect.
The construction of a conveying unit able to satisfy these conditions for relatively low working frequencies is generally no problem. The situation changes radically however when the working frequency exceeds a determined value, and becomes in fact prohibitive when the required working frequency is very high, as when feeding an intermittent packaging machine capable of a working frequency of the order of seven or more steps per second.
The inertia forces acting on the preshaped pieces at such speeds are such that not only must the preshaped pieces be constantly supported and guided during transfer from one conveyor to the other, but the mobile members concerned with this transfer must be of a very small number and simple structure both in order to allow them to be easily driven synchronously and so as not to require very complicated control equipment, which would considerably reduce the reliability of the conveying unit.